Superconductive conductors may be subjected accidentally or under special operating conditions to partial transitions, i.e. to a limited region of the conductor switching to the resistive state. This region is initially very small, but it grows progressively because of Joule effect heating. The intense and rapid heating and the high voltages that appear across the region that has changed state run the risk of damaging the conductor.
Superconductive conductors generally contain a large fraction of copper or of aluminum, which fraction is judged necessary for protecting them in the event of a transition. When cold, the resistivity of copper or aluminum is 1,000 to 10,000 times smaller than the resistivity of the superconductive material after transition or of a matrix alloy such as cupro-nickel, thus making it possible to reduce the Joule effect considerably and the heating that results therefrom for given current.
In general, the transition propagates slowly so the current falls off slowly as well, too slowly to prevent thermal damage. Various protection systems are therefore used.
Active protection devices are known having the function of detecting a transition and of acting on the power supply circuit so as to cause the current to decrease more quickly. However, the rate at which the current decreases is limited by the maximum voltage that can be accepted across the terminals of a superconductive coil.
In order to provide an improved system of protection against partial transitions, a passive protection system is proposed providing particularly high propagation speeds for the region that has changed state.